Issue 356 Add live updates to dashboard widget

Live Updates for Dashboard Widget.

-"Enable updates" checkbox
-Default setting to true on install
-Configure heartbeat response
-One method to fire on 'heartbeat_recieved' to handle Dashboard and Stream Page requests (DRY)
-One method to generate list items for dashboard widget, whether initial page load or heartbeat
-JS to handle updating the Dashboard
-When new items are added, remove correct number to keep "records per page"
-Only display updates on first page of results
-Highlighting/Fadeout of new items

Fixes https://github.com/x-team/wp-stream/issues/356

@fjarrett